The speeds of metal-oxide-semiconductor (MOS) transistors are closely related to the drive currents of the MOS transistors, which drive currents are further closely related to the mobility of charges. For example, NMOS transistors have high drive currents when the electron mobility in their channel regions is high, while PMOS transistors have high drive currents when the hole mobility in their channel regions is high. Germanium, silicon germanium, and compound semiconductor materials of group III and group V elements (referred to as III-V compound semiconductors hereinafter) are thus good candidates for forming their high electron mobility and/or hole mobility.
A current problem posted to using germanium, silicon germanium, and III-V compound semiconductor in the integrated circuit formation processes is the difficulty in the formation of the thin films of these semiconductor materials. Currently, there is no feasible bulk growth method. Therefore, germanium, silicon germanium, and III-V compound semiconductors are commonly formed by epitaxially growing films on substrates such as Si or SiC substrates. The existing available substrate materials, however, do not have lattice constants and thermal expansion coefficients closely matching that of III-V compound semiconductors. For example, the lattice constant of silicon is about 5.43 Å, the lattice constant of germanium is about 5.66 Å, while the lattice constant of GaAs, which is a commonly used III-V compound semiconductor, is 5.65 Å. As a result, the resulting germanium-containing semiconductors and III-V compound semiconductors grown from other substrates suffer from high defect densities. Various methods were thus explored to reduce the defect densities in the grown semiconductors. A known method is to form recesses in shallow trench isolation regions, and then grow the germanium, silicon germanium, or III-V compound semiconductors in the recesses. Although the semiconductors formed using this method generally have lower defect densities than what are grown from blanket silicon wafers, the defect densities were still often high.